The present invention relates to a flexible metal tube having a box element and to a flexible pipe comprising at least one such metal tube.
Coflexip, one of the assignees hereof, has for decades been manufacturing and marketing long flexible pipes of high mechanical strength, which can be used especially for transporting fluids such as hydrocarbons, in particular in sub-sea oil production installations.
Several types of flexible pipes are currently used. A first type of flexible pipe, called a xe2x80x9crough borexe2x80x9d, consists of an internal metal carcass, made from a crush-resistant doubly interlocking profiled metal strip, an inner sealing sheath, generally polymeric, at least one armour intended to give the flexible pipe a high-strength to withstand the internal pressure and the axial tension, and an outer sealing sheath, generally polymeric. One or more sealing sheaths, whether polymeric or not, may also be used, for example between armour plies, when the flexible pipe comprises several of them.
Flexible pipes called xe2x80x9csmooth boresxe2x80x9d are also used and comprise a sealing sheath as the innermost component.
Particularly high-performance doubly interlocking profiled metal strips are especially described in the patent FR 2 654 795 of Nov. 21, 1989 and have been used to produce an internal carcass for a flexible pipe, by making a flat metal tape, such as a stainless steel strip, undergo plastic deformation in order to give it the shape of a doubly interlocking profiled strip, and then by spiralling the profiled strip, that is to say winding it helically with a short pitch with interlocking of the turns of the profiled metal strips. After two consecutive turns have been interlocked, a final plastic deformation of the strip is carried out in order to complete the interlocking.
In Patent FR 2 665 237 it is recommended to produce a tubular metal carcass comprising at least one box element wound in a helix with a short pitch, the said metal carcass being obtained by means of two complementary profiled strips wound helically with a short pitch. Many examples of profiled strips are described and shown in that document, some of which, such as for example those in FIGS. 8 and 9, consisting of a strip in the form of an elongated S and having a box element at a first end and an upwardly curved fastening edge at the other end, the fastening edge penetrating a dish formed by the box element and the transverse bar of the S of the preceding turn. The curved fastening edge may rest on the bottom of the dish (FIG. 9) or it may not be in contact with the said bottom (FIG. 8).
It should be noted that all the cross-sections of the box elements provided at one or both ends i.e. side edges of each profiled strip are squire or rectangular cross-sections.
Although such box-element profiled strips have been satisfactory, they have been found to have certain drawbacks. When one considers that a box element is formed by parts of the same turn of the profiled strip and comprises an upper wall, a lower wall and side walls and when an external force is applied to one of the upper and/or lower walls, such as a compressive or crushing force or else a force generated by the pressurized fluid, buckling of the side walls or faces of the box element then occurs, thereby reducing, at least locally, the crush resistance of the internal carcass.
For forces or pressures exceeding a certain value, buckling of the side walls of the box element occurs.
For lower forces or pressures and when the side walls are not strictly perpendicular to the upper and lower walls of the box element, crushing of the said box element may occur, resulting in the side walls moving further apart or closer together (opening or closing of the box element). To prevent these opening or closing phenomena, it has been recommended in certain cases to weld one end of the box element at one point on the strip. However, such a solution increases the manufacturing cost and it is difficult to implement.
In addition, the relatively large interstices or interjoint spaces, which exist between two consecutive turns or between two adjacent parts of the same turn, such as those which are formed in a dish, promote the creep of the adjacent sealing sheath, this being something which it is desired to avoid as far as possible, mainly in the case of pressure vault.
The object of the present invention is to remedy the aforementioned drawbacks and to provide a flexible metal tube which can be used in a flexible pipe and which is more suitable for the applications for which it is intended.
The subject of the present invention is a flexible metal tube of longitudinal axis, which comprises at least one strip of approximately constant thickness wound helically about the axis, the strip having at least one closed box element whose walls are formed by portions of the same turn of the strip, the said box element having an upper wall and a lower wall and being provided at one first end i.e. lateral edge of the strip, the other second end i.e. lateral edge of the strip terminating in a free edge, the tube being characterized in that the box element has a trapezoidal cross-section and in that a bulge directed radially towards the upper wall of the box element is provided between the box element and the second free edge of the strip.
The trapezoidal cross-section of the box element makes it possible to have a deformation of the side walls or faces of the box element which is always directed in the same direction, with a tendency to open the box element.
The bulge directed radially towards the upper wall of the box element and provided between the box element and the free edge of the profiled strip has many advantages.
Firstly, the radial bulge or projection blocks the opening of the box element, when opening occurs, thereby stabilizing the geometry of the said box element.
Secondly, penetration of the free edge of the strip into the adjacent radial projection or bulge, of the next consecutive turn of the strip when the free edge is curved towards the upper face of the box element, interlocks two consecutive turns of the helical winding.
Finally, the dimensions and the positioning of the radial bulge on the profiled strip may be chosen so as to considerably limit any creep of a sealing sheath bearing on the said strip.